Large quantities of saw by-products accumulate in the wood-working industry. Among them are, above all, sawdust, wood shavings and wood chips, from which ordinarily wood pellets are made. One such rod-like wood pellet has, for example, a diameter ranging from 4 mm to 10 mm, a length between 5 mm and 45 mm, a bulk density from about 1.1 to 1.3 kg/dm3, and an approximate energy density of 5 kWh/kg. Pellet manufacturing is done in a known annular, circular or flat die press, in which Koller rollers press the material to be pressed through the bore holes or pressing channels of the die. This process is characterized by temperatures exceeding 120° C. and very high pressures, and they are highest in the wall areas of the pressing channels. Consequently, the cylindrical surface of the pellets is very smooth and has a very high density.
On the inflow side, the press channels have a conical inflow chamfer only a few millimeters deep. Looking closer at an individual press channel, one can see that the compacting of the fibrous and fine chip material takes place cyclically every time a Koller roller rotates past. The funnel-shaped fibrous structure in the pellets is created by the funnel-shaped chamfer. For this reason, pellets have a convex surface in the front and a concave surface in the back. A pellet die for 6-mm thick pellets has, with a press channel bore hole of 6 mm diameter, for example, a press channel length of about 40 mm and widens radially to the exterior in steps after 20 millimeters to 6.4 mm, 6.8 mm and 7.2 mm. The background for this is explained by the fact that the pressed strand of fibrous material should be guided even further after the actual press channel section with the 6-mm diameter and the die needs to have certain strength of material for reasons of resistance of materials. The front surface of the pellets has a highly irregular, rough and brittle surface. As a result of this, the front sides of the pellets are mainly responsible for the creation of troublesome fine particles and dust every time the pellets are manipulated.
Typically, pellets are injected by means of silo tankers in the storage bunker. It has been shown that the long pellets “fly” through the hose pipes at an angle of about 45° against the direction of the airflow in the middle of the blow pipe cross section almost without touching. Long pellets have better “aerodynamic properties” than short pellets. For this reason, during the injection process, the short pellets are found primarily on the bottom of the injection hoses together with the broken pieces, where they are largely transported in a rolling way. In this case, they make contact with the underside of the hose and collide with the house couplings, thereby producing more fine particles. Short pellets, pellet fragments and dust flow a lot worse than uniformly long pellets.
The front sides have a significantly higher friction in pellet bulks than the smooth cylindrical surfaces. The percentage share of front sides drops linearly with the growing average length of the pellets. The ideal length of pellets is about 25-35 mm for small pellet fuels. The maximum length of wood pellets has been fixed by the 7.5×D (diameter) formula established by the European Standard EN 14961-2. For the commercially available, small-fuel pellets having a diameter of 6 mm this translates into a maximum length of 45 mm.
In this regard, the cutting techniques usually employed in the market have some drawbacks. In circular dies, fixed transverse bars made of flat steel are arranged in the radially outer area at a distance. Initially, the pellet grows out of the fast-rotating annular die as it passes through the press channel. As soon as a pellet is long enough, it hits against one of the transverse bars while it circulates, in which case it is broken up by the annular die. The transverse bars made of flat steel are twisted 90° with respect to the circulation direction of the annular die. Consequently, the pellets are suddenly slowed down when they hit the transverse bar perpendicularly. The pellets are still very hot (just under 100° C.), mechanically unstable, and still soft. The path velocity of an annular die is about 7.5 m/sec. When they are being knocked off, the pellets are therefore accelerated strongly and set rotating by the standing transverse bar arranged at a 90° angle with respect to the rotational direction. Afterwards, they still hit more transverse bars or other structural parts of the pellet press at high speed. When the pellets are severed from the annular die and/or when they subsequently strike the other parts of the pellet press, they break up uncontrollably in pieces of varying length. A lot of dust created by the fragments is formed in the process. In an annular die having a hole diameter of 6 mm, the fine particles generated in accordance with the known cutting technique amount to about 5%-8%.